Generally, in an internal combustion engine provided with an exhaust purification system utilizing a catalyst, it is essential to control a mixing ratio of air and fuel of an air-fuel mixture burnt in the internal combustion engine, i.e., an air-fuel ratio, in order to perform purification of harmful components in exhaust gas by the catalyst with high efficiency. In order to perform such control of the air-fuel ratio, in the internal combustion engine as described above, sensors that generate outputs according to oxygen concentrations of the exhaust gas are provided at upstream and downstream sides of the catalyst of an exhaust passage, i.e., a catalytic purification device, and air-fuel ratio feedback control is carried out so that the air-fuel ratio is made to follow a target air-fuel ratio based on outputs of them. For example, a so-called wide-area air-fuel ratio sensor is provided at the upstream side of the catalyst, and a so-called oxygen sensor is provided at the downstream side thereof.
A general oxygen sensor is disposed in the exhaust passage so that an inner surface of a detection element of the oxygen sensor is exposed to the atmospheric air, and that an outer surface thereof is exposed to the exhaust gas, and when a difference is generated in oxygen partial pressures of the atmospheric air and the exhaust gas, in short, a difference is generated in oxygen concentrations, oxygen ions flow inside the detection element from a side with high oxygen concentration to a side with low oxygen concentration, and thus an electromotive force is generated. However, when a defect occurs in the detection element of the oxygen sensor, i.e., when an element crack occurs, the exhaust gas flows inside the detection element, and the difference is not generated in the oxygen concentration between inside and outside the detection element. As a result of it, the oxygen sensor generates an output similar to an output at the time of so-called lean combustion in which oxygen increases in the exhaust gas. That is, when the defect occurs in the detection element, a degree increases in which the oxygen sensor generates the output similar to the output at the time of lean combustion. Consequently, it is possible to detect that the detection element of the oxygen sensor has a defective abnormality based on a degree of an output tendency of a lean side of the oxygen sensor.
As described above, when the detection element of the oxygen sensor has the defective abnormality, the output of the oxygen sensor does not generally correspond to an oxygen concentration of the exhaust gas, and thus when the above-described air-fuel ratio control is performed simply based on the output of the oxygen sensor, emission is deteriorated. Particularly, since such an oxygen sensor tends to generate an output similar to the output at the time of lean combustion as mentioned above, correction to excessively enrich an air-fuel ratio may be performed based on the output of the oxygen sensor in the air-fuel ratio control.
For example, PTL 1 discloses an air-fuel ratio control device for preventing emission deterioration as described above. This device has a configuration that determines that a possibility of abnormality is higher as a degree is larger in which appearance frequency distribution of an output value of an oxygen sensor provided at a downstream side of an exhaust purification catalyst is shifted to a lean side, and sets a limit to a correction amount to air-fuel ratio control in a direction to suppress enriching of an air-fuel ratio according to a degree of the possibility of abnormality.